The present invention relates to power plants and pertains particularly to improved apparatus for improving mixing of sorbents with combustion gases in circulating fluidized bed (CFB) boiler plants to lower sulfur dioxide levels in the flue gas.
There exists in the power generating industry a need for more efficient power plants for converting fossil fuels to electrical power. The need is even greater for higher efficiency plants for converting lower grades of sulfur containing fuels, such as coal, that exist in abundance in many regions of the world.
Atmospheric pollution is of great concern throughout the world today. One of the major causes of atmospheric pollution is the burning of various fossil fuels for the generation of heat and power. Many of these fuels contain impurities, such as sulfur which reacts in the combustion process forming compounds such as SO.sub.2 that is particularly noxious and polluting. Systems, including scrubbers, have been developed for removing these pollutants from exhaust gases of power plants. However, these systems are very expensive and frequently not cost effective for most power plants.
Circulating fluidized bed combustors have been developed in recent years for burning low quality fuels, such as coals, for generating steam for powering steam turbines. The circulating fluidized bed combustor features a mixture of granular limestone or other sorbent materials supported on a non-sifting grid. An upward flow of air passes through the grid lifting and fluidizing the material. This results in a turbulent mixture of the bed particles having the free flowing properties of a liquid and providing an environment for stable combustion. Fuels introduced into the bed will burn effectively, and sulfur dioxide released by the burning is chemically captured by the calcined limestone. The mixture of solids which includes ash and calcined limestone is recirculated through the combustor until the particle size is reduced sufficiently for elution through the cyclones.
As sulfur containing fuel is burned, the sulfur volatilizes under the high temperatures and combusts with oxygen to form sulfur dioxide. The limestone is calcined by the combustion temperatures, and the sulfur dioxide then reacts with the calcium oxide and oxygen to form calcium sulfate. The oxygen and sulfur are contained in the gas of the fluidized stream, and the calcium oxide is contained in the particles. Sulfur removal depends on contact between the sulfur dioxide molecules and the calcium oxide particles.
Recent studies have shown that as combustion and recirculation occurs the calcined limestone particles are transported up through the combustor with little or no turbulence except near the walls of the combustor. The sulfur dioxide was found to be more highly concentrated in the center of the combustor, with lowered sulfide dioxide levels in the flue gas near the combustor walls. This phenomenon has been described as a "sulfur dome" when traverse readings are taken across an operating CFB combustor. This distribution leads to the requirement of more transverse circulation of the particles and combustion gases in the combustor to lower sulfur dioxide levels in the flue gas.
The calcium sulfur ratio (Ca/S) required for a desired amount of sulfur removal is a function of how much excess particle density in the gas stream is required to insure that a sufficient number of sulfur dioxide molecules come in contact with the calcium dioxide particles. It is, therefore, desirable to improve the contact between the calcium and the sulfur dioxide particles.
Applicant has discovered and developed an arrangement whereby a circulating fluidized bed (CFB) for burning sulfur containing fuels is made to utilize limestone more efficiently by the injection of high velocity steam into the circulating fluidized bed (CFB) boiler to improve the mixing of the recirculating solids with the combustion gases.